1. Field of the Invention
The present invention relates to a printed circuit board with characters printed on its front and back faces and to a method for the fabrication of the printed circuit board.
2. Discussion of Background
Generally, printed circuit boards are assembled, modified, inspected or adjusted in accordance with the following procedures which take place very frequently. That is, a specific terminal, pattern, or through hole recognized on the front face of a circuit board is required to be found on the back face of the circuit board for its modification or for connecting the probe of a measuring device to it. It is the most time-consuming work to find such a specific position of the front face on the back face of the circuit board. This is because conventional printed circuit boards having cirucit patterns, characters, and symbols printed on their front faces have different circuit patterns, characters and symbols on their back faces. Consequently, to find a position common to the front and back faces of the circuit board it was necessary to use as an index the relation between the position of characters and symbols printed on the front face and that of characters and symbols on the back face. In other words, the common position had to be found depending on an intuition and a "location" which will be described hereinafter.
The above mentioned printing of characters are effected by the method of silk-screen printing. In the silk-screen printing, the patterns of characters and symbols drawn on a negative are photoprinted on a screen such as silk or nylon having numerous meshs or on a stainless steel mesh to prepare a silk printing plate. With this silk printing plate, the characters and symbols are printed on a substrate. The above negative is prepared by manually writing, applying a tape, or printing desired characters and symbols on a transparent sheet. This negative is prepared by forming the characters and symbols by an area which passes light and an area which does not so that the above silk printing plate is prepared by photoprinting.
Accordingly, the conventional silk-screen printing employes a negative shown in FIG. 1(A) to print the characters on the front face of a substrate and a negative shown in FIG. 1(B) on the back face of the substrate. The characters printed by the use of these negatives represent normal images on the front and back faces of the circuit board. Therefore, the characters printed on the printed circuit board do not make indexes which are used to locate a particular position on the printed circuit board. For example, the terminal "+" of an electrolytic capacitor C4 illustrated in FIG. 1(A) is found positioned just below the character "4" of the characters "C4" when observed on the front face, but it is also found just below the character "C" instead of the character "4" when observed on the back face as illustrated in FIG. 1(B). Consequently, when any printed character is used as an index to find a certain position common to the front and back faces of the circuit board, a different position is often located in error. Besides, it takes a long time and a lot of trouble to find such a position.